The present invention relates to a method of cooling, particularly during the quenching operation, the outer surface of large diameter metal pipes from 18 inches to 50 inches or more in diameter. More particularly the invention relates to a cooling method for such pipes which is capable of reducing the amount of uneven circumferential cooling of the pipe when the pipe which has been heated to its quenching temperature in a high-frequency induction electric heating unit or the like is rapidly cooled while the pipe is being passed through the rings.
In the known methods of the above type such as shown in FIG. 1 of the drawings, the rapid cooling of a large diameter metal pipe for quenching purposes is accomplished by spraying a coolant, e.g., water from a ring header 1 against the pipe. In this case, the usual practice is to spray water against the pipe surface to be cooled in such a manner that the water jets are directed toward a central axis 3 of the pipe with a certain dip angle and in a direction opposite to a heating unit 4. With these conventional methods, however, the careful consideration of the jet velocity or the dip angle of coolant such as water is for the most part omitted, with the result that not only the sprayed coolant is not necessarily utilized effectively for cooling the pipe, but also there results considerable variations in the circumferential cooling rate of the pipe. As a result, a strain is caused in the cooled pipe thus presenting a serious problem. This strain consists of a thermal strain caused by variations in the cooling rate at different parts of the pipe during the cooling or the thermal strain plus a transformation strain and thus it is essential to uniformly cool the entire pipe from the beginning of the cooling operation to the transformation completion temperature (e.g., from about Ac.sub.3 to 400.degree. C. in the case of steel pipes). Where the wall thickness of a pipe to be cooled is large or the travel speed of the pipe is high, the conventional methods usually arrange at least one header 2 of the same construction (as to the spray angle and direction) in the rear of the first stage header 1 as shown in FIG. 1, since the cooling of the pipe from the cooling starting point to the desired cooling complete temperature cannot be effected within a predetermined time with a single cooling header. In the case of the conventional methods, irrespective of the number of the headers used, the sprayed cooling water flows down from the upper portion to the lower portion of a pipe along its wall. Thus, despite the fact that the coolant is uniformly sprayed circumferentially against the pipe, the cooling effect on the lower pipe portion is increased by the above-mentioned reasons, with the result that the upper and lower pipe portions are unevenly cooled and the resulting thermal or transformation strain or both cause the long pipe to bend or become out-of-round.